1. Field of the Invention
The present invention relates to a fuel cell stack having stacked unit fuel cells, each having a structure in which an anode and a cathode are provided on either side of an electrolyte membrane, and each unit fuel cell is placed between separators.
2. Description of the Related Art
Typically, solid polymer electrolyte fuel cells have a unit fuel cell in which an anode and a cathode are provided on either side of an electrolyte membrane consisting of a polymer ion exchange membrane (i.e., cation exchange membrane). The unit fuel cell is placed between separators which are provided for supporting the unit fuel cell. Generally, a specific number of unit fuel cells are stacked to obtain a fuel cell stack.
In this kind of fuel cell stack, a fuel gas supplied to the anode, such as hydrogen, is ionized to hydrogen ions on catalytic electrodes, and the hydrogen ions are transferred to the cathode via an electrolyte membrane which is humidified to have an appropriate level of humidity. During this process, electrons are generated and flow to an external circuit, providing DC (direct current) electrical energy. An oxidizing gas such as oxygen or air is supplied to the cathode, and the hydrogen gas, electrons, and oxygen gas react at the cathode, thereby generating water.
Japanese Unexamined Patent Application, First Publication No. Hei 8-171926 discloses an example of the fuel cell stack. In the fuel cell stack, plural sets of the unit fuel cell and the separators are stacked to have a stacked body, and electrical power is drawn from terminals provided at either side (in the stacking direction) of the stacked body. In the disclosed system, a plurality of such stacked bodies are provided, and a member for supplying and discharging fuel or the like is provided between adjacent stacked bodies. In addition, a pressing mechanism for pressing the adjacent stacked bodies (between which the above-explained member is provided) from either side of the stacked bodies towards the center thereof is further provided.
When the above fuel cell stack is assembled, the member for supplying and discharging fuel (or the like) and the pressing mechanism are first placed in the horizontal direction, and then the unit fuel cells are stacked between this member and the pressing mechanism. This process of stacking the unit fuel cells and separators in the horizontal direction has a problem in that it is difficult to precisely position the adjacent unit fuel cell and separator, or the adjacent separators with each other.
Conversely, if the unit fuel cells and the separators are stacked in turn in the vertical direction, the adjacent unit fuel cell and separator, or the adjacent separators can be easily and precisely positioned in the assembly process. For example, as shown in FIG. 7, one of end plates 102 is stacked via a cushioning member 101 on one of horizontally-laid backup plates (i.e., fastening plates) 100, and on this end plate 102, an insulating plate 103 and a terminal plate 104 are stacked. All the unit fuel cells 105 and separators 106 and 107 are stacked on the terminal plate 104, in a manner such that each unit fuel cell 105 is located between the separators 106 and 107. The other end plate 102 is further stacked via a terminal plate 104 and an insulating plate 103 on these stacked unit fuel cells and separators, and on this end plate 102, the other backup plate (i.e., fastening plate) 100 is further stacked via disc springs 108. This stacked body is fastened using the bolt members 109 from the outside of either backup plate 100, so as to obtain an assembled body.
In comparison with the horizontally-stacked type, the above fuel cell stack has an advantage in that the adjacent stacked unit fuel cell 105 and one of the separators 106 and 107, or the adjacent separators 106 and 107, can be precisely positioned with each other. However, all the unit fuel cells 105 and separators 106 and 107 are stacked on a single reference base which is one of the backup plates 100; thus, errors in shape (i.e., deformation) of the unit fuel cells 105 and the separators 106 and 107 are cumulative; thus, the stacked body 110 may bend in the stacking direction. Therefore, when another backup plate 100 is stacked on the deformed body and the backup plates 100 are fastened together by using the bolt members 109, the fuel cell stack itself may bend in the stacking direction. If such a fuel cell stack having a bent shape is built into a vehicle, the fuel cell stack may interfere with other structures.
In consideration of the above circumstances, an object of the present invention is to provide a fuel cell stack for reducing the bending of the stacked body in the stacking direction, thereby preventing the interference between the stacked body built into a vehicle and other structures in the vehicle.
Therefore, the present invention provides a fuel cell stack comprising:
a stacked body (e.g., stacked body 17 in an embodiment explained below) having a plurality of stacked unit fuel cells (e.g., unit fuel cells 14 in the embodiment explained below), each unit fuel cell being placed between and supported by a pair of separators (e.g., separators 15, 16 in the embodiment explained below), wherein each unit fuel cell has an anode (e.g., anode 12 in the embodiment explained below), a cathode (e.g., cathode 13 in the embodiment explained below), and an electrolyte membrane (e.g., solid polymer electrolyte membrane 11 in the embodiment explained below) which is placed between the anode and the cathode;
fastening plates (e.g., backup plates 29 in the embodiment explained below) provided at either end of the stacked body in the stacking direction of the stacked body;
an intermediate plate (e.g., intermediate plate 39 in the embodiment explained below) provided at an intermediate position of the stacked body in the stacking direction; and
bolt members (e.g., stud bolts 19 in the embodiment explained below) inserted through the intermediate plate in the stacking direction in a manner such that the movement of the bolt members with respect to the intermediate plate in the direction perpendicular to the stacking direction is restricted so as to fix the relative position between the intermediate plate and the bolt members in the relevant direction,
wherein the bolt members are also inserted through the fastening plates in the stacking direction, and the stacked body is fastened together by fastening the fastening plates towards the center of the fastening plates by using the bolt members.
As a typical example, the intermediate plate is provided approximately at the center of the stacked body in the stacking direction.
The bolt members may also be inserted through the stacked body in the stacking direction.
The above structure provides the intermediate plate at an intermediate position of the stacked body in the stacking direction, in other words, the stacked body including the stacked unit fuel cells and separators is divided into two portions located at either side of the intermediate plate. Therefore, cumulative errors in shape in the direction of the thickness of the stacked body (i.e., the unit fuel cells and separators) can be substantially reduced to half. Accordingly, the bending of the stacked body in the stacking direction can be considerably reduced, and the interference between the fuel cell stack built into the vehicle and any other structure in the vehicle can be prevented. The present invention is especially effective when the separators are made by press forming, where errors in shape tend to occur in this case.
In addition, the bolt members are inserted through the intermediate plate in the stacking direction in a manner such that the movement of the bolt members with respect to the intermediate plate in the direction perpendicular to the stacking direction is restricted, and the stacked body is fastened together by fastening the fastening plates towards the center of the fastening plates by using the bolt members. Therefore, after the fastening process, the movement of the intermediate plate with respect to the bolt members in the direction perpendicular to the stacking direction is restricted, thereby preventing a shift of the intermediate plate due to vibration or the like.
Therefore, in comparison with structures in which the movement of the intermediate plate with respect to the bolt members is not restricted, undesirable shifts of the intermediate plate can be avoided. In addition, the bolt members inserted into the intermediate plate can function as a guide for stacking the unit fuel cells and the separators, thereby improving the working efficiency in the stacking process.
Typically, each bolt member has a fitting portion; and the intermediate plate has a fitting hole into which the fitting portion of the bolt member is fit.
Preferably, the movement of the bolt members with respect to the intermediate plate in the stacking direction is also restricted so as to fix the relative position between the intermediate plate and the bolt members in the relevant direction.
In this case, typically, each bolt member has a fitting portion; and the intermediate plate has a fitting hole into which the fitting portion of the bolt member is fit, wherein the fitting portion has a flange portion and the fitting hole has a corresponding step portion so as to restrict the movement of the bolt members with respect to the intermediate plate in the stacking direction.
It is possible that a plurality of intermediate plates through which the bolt members are inserted are provided, wherein the movement of the bolt members in the direction perpendicular to the stacking direction is restricted so as to fix the relative position between the intermediate plates and the bolt members in the relevant direction.
In this case, preferably, the movement of the bolt members with respect to the intermediate plates in the stacking direction is also restricted so as to fix the relative position between the intermediate plates and the bolt members in the relevant direction.